EFFECTS OF NITROGEN ON THE RESPONSE OF LOBLOLLY-PINE TO DROUGHT .2. BIOMASS ALLOCATION AND C-N BALANCE

Effects of drought on carbon and nitrogen dynamics and biomass allocat ion patterns of loblolly pine (Pinus taeda L.) seedlings grown under 4 different nitrogen levels were determined. Nitrogen stress caused an increase in root-shoot ratio, increased foliar and root starch concent ration, decreased soluble sugar concentration, and decreased free amin o acid concentration in well-watered seedlings. Drought generally caus ed an increase in foliar hexoses and decreases in foliar and root star ch and root hexoses, while not affecting foliar free amino acids. Howe ver, nitrogen stress and drought interacted in affecting carbohydrate pools and seedling morphology. Foliar starch reserves in high-N seedli ngs were rapidly metabolized under drought, while N-deficient seedling s maintained similar starch concentrations throughout the drought stre ss cycle. After one drought cycle, the N-induced difference in root:sh oot ratio was no longer significant. These results suggest that W-stre ssed seedlings partition a higher percentage of fixed carbon to storag e, and that this storage pool is less available for immediate use than in seedlings grown under adequate N: